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Castimir was raised in Rimmington. There, he spent a nice childhood filled with such activities as attacking the nearby giant rats with sticks and stones with his friend Theodore and experienced a madman raving about the lost city of Zanaris and the Fairy Queen. In his teenage years, Castimir's uncle attempted to get him to become an apprentice crafter to Rommik, though this quickly ended after he failed to make a gold ring, causing much destruction and scalding Rommik. Some time later, a wizard spotted Castimir's potential as a wizard and gave him some runes to cast a fire strike to prepare for the Wizards' Tower entry exams. Instead of killing a rat, Castimir accidentally set fire to Rommik's Crafty Supplies.

After the incident, Castimir completed his entry exams and departed on his one year adventure. He made a brief trek into Kandarin, crossing over White Wolf Mountain with a group of travelers. Near the end of the journey through the mountain, his mule was injured by a wolf and dies, giving Castimir reason to buy a yak from a Fremennik fur trader.

Nearing the end of his year-long journey, Castimir headed back to the Wizards' Tower, stopping at Taverley where he met Ebenezer and Gar'rth. He was also reunited with Theodore, now a squire with the White Knights. After a brief catching up with his childhood friend, Castimir joins Ebenezer and Gar'rth as they travel to the Monastery to cure Gar'rth of his lycanthropy. Stopping at Gunnarsgrunn, Castimir meets the barbarian priestess and his future lover, Arisha, who was also heading to the Monastery, Kara-Meir who was running away from Falador, Doric and Squire Theodore, who were chasing after Kara-Meir.

Castimir proves his skill as a wizard during the Battle of the Monastery, where by working with Ebenezer he is able to wound Jerrod and aid in the escape of his companions and the monks.

During the Siege of Falador, Castimir aided the agents of the Wizards' Tower in defending the city, and was one of the few mage survivors of the battle.

After the Siege of Falador, Castimir took Master Segainus' tomes of knowledge and planned to head to the Wizards' Tower to complete his training. He planned to meet up with his friends at Varrock on Midsummer's Eve. He was named one of Ebenezer's heirs. He also planned to write to Arisha because "she still has my yak...".

Castimir travelled to Varrock meeting Gideon Gleeman, the court jester, on the way. He turned in his thesis to Aubury and barely passed, being given a beginner wand for his promotion. During the festivities of Midsummer's Eve, he helped save the baby Felicity from the Wyrd, and saved Gar'rth when Captain Rovin attacked him, much to the anger of Aubury. For this he was sent as part of the embassy to Morytania to bid for the return of the captured children. Despite his best attempts, Aubury caught up with him before he left, and made it clear that the attack on Rovin may very well end is career as a wizard. While in Canifis, he discovered that the werewolves have a massive stock of runes as well as a tome similar to those held by Master Segainus. He offered to buy the book from the wolves, but Roavar refused, claiming that to do so would violate the pact the blood mark had guaranteed. During the initial confusion of the attack on Canifis by the Myreque to free Vanstrom Klaus, he accidentally stole the book. He was captured alongside Gar'rth, Theodore and Kara-Meir and taken prisoner by The Black Prince, but was released shortly after. He managed to survive the attack launched by Vanescula Drakan to kill all of the Myreque from Hope Rock and escaped across the river Salve.

Following his return from Morytania, he was escorted to Varrock by Captain Hardinge, where he was arrested by the Wizards' Tower for the theft of Master Segainus' tomes and hiding of knowledge, considered a very serious crime by the Tower, and beaten by Captain Rovin. He was held prisoner in bad conditions until his trial before Aubury, Seridor and Grayzag. He was found guilty of theft and hiding knowledge and expelled from the Tower by a unanimous vote, but was spared execution by Aubury, who voted against it and thus denied Grayzag the majority needed to execute criminal wizards.

Castimir became a rogue mage, secretly employed by the Tower to carry out darker operations that regular members of the Tower could not be seen doing. After Kara-Meir foiled an assassination attempt on Arisha's life, her romantic relationship with Castimir was uncovered. When the veil separating Morytania and Misthalin at Paterdomus began to weaken, Castimir travelled with Arisha to Gunnarsgrunn in order to find out how to obtain the rune essence needed to strengthen the veil. There, the barbarians despite expectations accepted Castimir and his relationship with Arisha. Assassins again attacked Arisha, this time successfully poisoning her. Castimir travelled to the Mountain of Fire, home of the Untainted, a group of ancient barbarians who had taken part in the barbarian-wizard wars before becoming guards of the rune essence. Arisha was taken into the care of Magnus, a healer among the Untainted, while Castimir and Krum went in search of the pure essence. After Arisha and Krum stole some essence and ran off, Castimir hid among the raiders sent after them. He managed to meet up with them, as well as a group of Kinshra led by Lord Darquarius and Misthalin guards led by Captain Hardinge and Kara-Meir. Arisha gave herself up to the Untainted in exchange for letting the rest go.

Castimir, along with the rest, hurried to Paterdomus so that the veil could be strengthened. He joined in the battle against the zombies, and managed to survive until Tenebra was killed through the combined efforts of his son Gar'rth and Kara-Meir. He was grieved to hear about Ebenezer's demise during the Battle of King Botolph's Tower. He joined Gar'rth, Theodore, Doric and Kara-Meir in a voyage to Kandarin, determined to rescue Arisha.

It has been shown that Castimir is highly skilled in magic, capable of using high-level spells like Snare on Jerrod the werewolf, or possibly even Superheat Item as he demonstrated on breaking the lock of a door in the Monastery during the Battle of the Monastery. He is also shown to be able to come up with cunning plans that are able to deceive even the most cunning of enemies.

He is also able to fish, a skill he shared with Theodore, his childhood friend from when he was living in Rimmington.

On the other hand, Castimir was not skilled at Crafting, and was known to break whatever he tried to craft, much to his tutor Rommik's displeasure.

Castimir and Arisha have romantic interests in each other throughout the novels, but it is not until Legacy of Blood that their relationship becomes known. How long their relationship has been going on for is unknown. Following his expulsion from the Tower, Castimir impregnates Arisha, though he does not know it. Despite the tension between barbarians and wizards, which would normally make their love impossible, Castimir is welcomed among the barbarians for his relationship with Arisha, though Kuhn claims fathership of Castimir's unborn child to protect Arisha.

Throughout Betrayal at Falador, Castimir and Ebenezer had a debate about which is better: magic or science. Castimir defended magic and Ebenezer defended science. Despite this, they remained good friends. He was named one of Ebenezer's heirs following the Siege of Falador.

During Castimir's childhood in Rimmington, he became good friends with Squire Theodore, who he nicknamed "Theo". Following his expulsion from the Wizards' Tower, he blames Theodore for ruining his career and acting in a selfish manner to preserve his honour. They make up for it later, but their relationship is still strained.

Due to a number of incidents such as Castimir breaking a large quantity of items while making a gold ring in order to become a crafter and setting fire to Rommik's Crafty Supplies, Rommik holds a grudge against Castimir.